Feedback system testing apparatus



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4 Sheets-Sheet 1 C. F. WHITE FEEDBACK SYSTEM TESTING APPARATUS May l,1962 Filed July 11, 1958 RMAL PUT NO IN 8 2J+ il. lllL L AT U 2 NWP oumm l lllJ +Z+ w l I l l l May l, 1962 c. F. WHITE 3,032,710

FEEDBACK SYSTEM TESTING APPARATUS Filed July ll, 1958 4 Sheets-Sheet 21- L-r r) 'r INVENTOR CHA R LES F. W H lT E BY M/ ATTORNEY m Iii-)INVENTOR May 1, 1962 c. F. WHITE FEEDBACK SYSTEM TESTING APPARATUS FiledJuly 11, 195e 4 Sheets-sheet s CHARLES F. WHlTE (LOG SCALE) ATTORNEYScu2 w RAD/SEC. (Los SCALE) 0.5 2.o RAD/SEC. (Los SCALE) May 1, 1962 c.F. WHITE FEEDBACK SYSTEM TESTING APPARATUS 4 Sheets-Sheet 4 Filed Julyll, 1958 OCTAVES lll CU2:

DEPARTURE OF 2 'I CORNER FREQUENCY FROM CORRECT VALUE,

lE- LL m5 .23@ Summe@ 55E mmm lNvENToR CHARLES F. WH|TE ATTORNEYj UnitedStates Patent C) 3,032,710 FEEDBACK SYSTEM TESTING APPARATUS Charles F.White, Oxon Hill, Md. Filed .Iuly 11, 1958, Ser. No. 748,998 13 Claims.(Cl. 324-57) (Granted under Title 35, US. Code (1952), sec. 266) Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

The present invention relates to a method of and apparatus for testing afeedback system designed as an errorclosing closed-cycle systemhereafter referred to as a servo-system, which yields informationnormally obtained by open feedback loop tests, yet does not involveopening the loop and in some cases does not involve removing the systemfrom service.

In general a servo-system is a control device which produces an outputcontrol signal dependent upon an input or command signal, and contains a'self-correcting or feedback loop arrangement from the output to theinput. A detailed analysis may be found in R. E. Graham, Linear ServorTheory, Bell System Technical Journal, 1946, or L. A. MacColl,Fundamental Theory of Servomechanisms, D. Van Nostrand Company, Inc.,1945. The signals may be electromagnetic, mechanical, hydraulic,pneumatic, etc., or any of these used in combination.

The number of servo-systems used both for military and industrialapplications has increased rapidly in recent years. Some military usesare, for example, automatic radar range tracking systems, automaticangle tracking systems, regulated power supplies, and automatic pilots.Industrial uses of servo-systems include automatic processing systemsand product packaging control systems.

Since the components of most servo-systems are subject to variation dueto errors in manufacture, Wear, damage, or maladjustment, it iscustomary to test such equipment as it is produced and periodicallythereafter. In high performance systems, the usual closed-loop input andoutput measurements ordinarily are not satisfactory, because widevariations in the components of a system causing important deviations inperformance generally -are difficult to detect in closed-loop overallperformance measurement. For this reason and because system design isordinarily accomplished using the open-loop transfer function, it isdesirable to obtain an open loop transfer function, which heretofore hasnecessitated opening the feedback path of the servo loop.

There are, however, several reasons why it is generally diicult or notfeasible to make open loop tests. In some cases the system must besealed or so located that the loop is inaccessible. In other situations,as for example, steam turbine regulators, lthe system is unstable untilthe loop is closed. In still other cases, as for example, chemicalprocess control, an inferior product is produced during the period whenthe loop is open. In these and many other cases closed-loop testing iseither mandatory or highly desirable.

One object of the present invention is therefore, to provide a methodand apparatus for determining the openloop transfer function by tests ofa closed loop servosystem.

Another object of the present invention is to provide a method andapparatus, with functions as set forth above, and which requires accessonly to the input and output of the servo-system during performance ofthe test.

Another object of the present invention is to provide a method andapparatus with functions as set forth above, wherein the input signalapplied to the servo-system may icc be given a predetermined formwithout comprising the test result.

Another object of the invention is to provide a method and apparatus,with functions as set forth above, wherein a minimum number of activeelements is required.

Another object of the invention is to provide a method and apparatus,with functions as set forth above, wherein the required test signal isless complex than the signal which normally operates the system.

Another object of the invention is to provide Ia method and apparatus,with functions as set forth above, wherein the servo-system may betested While in operation.

Another object of the invention is to provide a method and apparatus,with functions as set forth above, wherein the components ofaservo-system may be compared with different, but functionallyequivalent, components in a test apparatus.

These and other objects will be better understood from the followingspecification and drawings in which like numerals in various figuresindicate like elements.

In the drawings:

FIG. l(a) shows one embodiment of the invention employing theservo-system error signal in the monitoring arrangement;

FIG. 1(1)) shows a second embodiment of the test apparatus employing theservo-system feedback signal in the monitoring arrangement;

FIG. 1(c) shows a third embodiment of the test apparatus employing amonitor arrangement suitable for a certain class of servo-systems;

FIG. 1(d) shows a fourth preferred embodiment of the test apparatusemploying the servo-system input and output signals;

FIG. 2 is a detailed embodiment of a summing point known as a signaladder,

FIG. 3 is a detailed embodiment of a difference-taking summing pointknown as an error detector;

FIG. 4 is a fth embodiment of the test apparatus for use withservo-systems having a complex input signal;

FIG. 5 is a sixth embodiment of the test apparatus for a certain classof servo-systems, which can be substituted for the test apparatus ofFIG. 4;

FIG. 6 is an alternate embodiment of a portion of the test apparatus;

FIG. 7 is an embodiment of a test signal source for use with theapparatus in testing a certain class of servo-systems;

FIG. 8 is a servo-system and a test apparatus of the invention producedusing standard analog computer techniques;

FIGS. 9(a) and 9(b) show asymptotic segment representations of the looptransfer functions of the test apparatus and the servo-system,respectively, in FIG. 8;

FIG. l0 is a matrix of test results produced by the test apparatus ofFIG. 8 for various servo-system gain and corner frequency departuresfrom specifications;

FIG. l1 is a test apparatus and servo-system of the type shown in FIG. 8wherein the test apparatus is constructed with passive network elements.

The subject matter pertaining to FIG. 6 is included herein to provide abetter understanding of the invention, but any rights thereto in thepresent disclosure have been disclaimed.

This subject matter has been incorporated in a copending patentapplication Serial No. 837,563, filed by Laurence F. Gilchrist, who wasoriginally listed as a coinventor in the present case. Briefly, thisinvention resides in methods and apparatus for generating a servo-systemtest signal and various methods and apparatus for test monitoring, whichin proper combination, provide the open loop transfer function of aservo-system without physically opening the loop during test. Thisinvention in addi- 3 tion provides methods and apparatus for generatinga test signal for application to certain classes of servo-systems whilein service and monitoring the result without disturbing the normaloperation of the system.

Referring specifically to FIG. l(a) there is shown a block diagram of aconventional servo-system 2 having a forward path 3 with an output overinput transfer ratio ,a and a feedback path 4 with a transfer ratio orfunction ,3. A signal applied to the input 5 is combined with the signalfrom the feedback path 4 by the summing point 6 to produce an errorsignal e at point 7. Signal e has a value equal to the input signalmultiplied by 1 l-M The output signal 0 at point Sis L 1-.u times theinput signal.

In order to measure the open-loop transfer function, a, of such asystem, it has been necessary to open the loop at some point. Openingthe loop, however, is not always possible, either because of physicallimitations or because the system will not operate satisfactorily. Thereis, therefore, a need for a testing method which will yield theinformation gained from an lopen loop test without opening the loop.

In the present invention a multiplier means is connected to a source oftest signal to multiply the test signal by l-,uT/ST, where ,aT and T aredesigned to equal ,u and respectively, and the resultant signal appliedto the servosystem input. The error signal should, therefore, be equalto the test signal, producing the same error signal conditions obtaininghad the test signal been applied directly to the open loop system. Themultiplier means may contain calibrated devices for varying ,LT and ,STto measure the Value of ,u and when the test method is employed as ananalyzer, but alternately, ,MT and 13T will be used as standards whileadjusting ,u and until the error signal e corresponds to the test signal0T (within acceptable go-no-go limits).

One arrangement 9 for performing this multiplication is shown in FIG.1(a). The test signal 0T from source 10 is applied to the junction 11.From junction 11 the test signal is passed through two paths 12 and 13to a summing point 14. The path 12 may be a direct connection allowingthe test signal to pass unaltered to the summing point 14 and is,therefore, designated by a unity transfer function. The path 13 containsmultiplier means 15 for multiplying the signal by the transfer functionMT followed by a multiplier means 16 for further multiplying the signalby the transfer function ,BT. The net effect in path 13 is to produce,at the summing point 14, a signal -Wrn While FIG. l(a) shows twoseparate means for multiplying 0T by ,aT and [iT, the required structureis merely a means for multiplying 0T by the product nT/ST and thedrawing could have shown a single means for this function.

FIG. 2 shows one embodiment of a summing point 14 of FIG. l(a) when theoutputs of paths 12 and 13 are electrical signals. Two resistors 21 and22 of substantially equal value are connected at one end to form ajunction 23, the ends 24 and 25 are connected to output ends of paths 12and 13, respectively. A much smaller resistor 26 having a value, forexample, one-thousandth that of resistor 21 has one end connected to thejunction 23 and its opposite end connected to a common return. Theoutput ends of paths 12 and 13 are thus effectively isolated and acombined output may be extracted between junction 23 and the commonreturn. The circuit of FIG. 2 is an adder type summing point, since itcombines the signals in phase. Similar results are obtained by applyingthe signal from each path to a different grid of a mixer tube.Mechanical systems, on the other hand, may require a differentialplanetary gear arrangement. Other equivalent arrangements will beobvious to those skilled in the art.

The signal from the output of summing point 14 in FIG. l(a) is appliedto one input of another adder type summing point 20 serially connectedbetween the normal input command signal source 27 and the input 5 of theservo-system. The purpose of this second summing point s to isolate thetest signal source and normal input signal source, thereby permitting asystem test during quiescent periods of 0T, i.e., periods when 0n isessentially zero.

To monitor the results of the test, the error signal in the servo-systemis compared directly with the signal from the test source. The testsource signal Waveform is completely arbitrary. If the test signalsource waveform is repeatable to close tolerances as in the case of asimple step function, for example, yobservation of the error signal, perse, may be sufficient. For a more complicated signal from a less stablesource it will be preferable to display the difference between theservo-system error signal and the test signal 0T, provided by a monitorto be described. As will be evident later, the feedback or the outputsignal may be used in a similar monitor.

A monitor for use with an unspecified test signal is provided bybridging a subtracter type summing point 17 between a first monitorpoint 7 in the servo-system and a second monitor point 11 in themultiplying arrangement 9. The output of the subtracter summing point isfed to indicating means 18. The function of the subtracter summing pointis to derive a signal equal to the difference between the signals fromthe first and second monitor points.

A typical form of a subtracter summing point or error detector for anelectrical servo-system is shown in FIG. 3. Points 7 and 11 of FIG. l(a)are connected to points 30 and 31, respectively, of FIG. 3. Amplifiers32 and 33 may be D.C. operational amplifiers of the type described in A.S. Locke, Guidance, D. Van Nostrand Company, Inc., 1955, chapter 19. Theresistors are all of equal value and may conveniently be of the order ofone megohm. Each amplifier has an odd number of phase-invert ing stagesto produce in combination With its input and feedback resistors a gainessentially equal to -1. 'Thus it can be seen that the signal at point30 is multiplied by -l before it reaches point 36. The signals at point31 and point 36 are each multiplied by -1 and thus the signal at point35 is the difference between the signals at point 30 and at point 31,respectviely. Numerous other means are well known to those skilled inthe art for performing the function of the subtracter summing point.Again in mechanical systems differential gears, etc., may be useddepending on the nature of the signals to be combined.

The signal applied to the indicating means 18 in FIG. l(a) is equal tothe test source signal 0T multiplied by the ratio of the sum of thequantities -f/.TT and ,a divided by the quantity (1-M/3). In allembodiments, described herein, of the means for developing a signal tobe observed by the indicating means, the type of summing point and theorder of connection in the case of sub` tracter summing points is chosento give results consistent with the above equation. If the values of they. and /3 transfer functions of the servo-system agree with thespecification values ,1LT and T, there will be zero output from themonitor. If a difference in these functions exists, the magnitude andpolarity of the monitor output as a function of time will indicate thetype and degree of difference. In studies of a particular type ofservo-system, the waveform of the monitor output signal may be used ininterpreting differences in gain and frequency response existing betweenthe functions ,TT and a.

It would be futile to attempt to illustrate all of the com-k binationsof elements which are covered by the n, #T and 18T portions of thesystems shown in the drawings. The ,u portions could contain vacuum tubeamplifiers, signal controlled motors, or complicated hydraulic systems,-

to mention a few of the possibilities. The ,8 portions may consist ofdirect electrical wire connections or a complicated system of activeand/ or passive elements. The MT and -T portions may be physically exactrduplicates of the p. and portions (except for a polarity inversion ofor they may be analogs. The -T in the multiplier arrangement is apositive transfer function since the servosystem is always basicallynegative. The means for producing ,HT, therefore, does not require aphase inverter. Means may be inserted in the monitor circuit, ifnecessary, for converting mechanical signals to electrical form, forexample, or the reverse, according to principles well known in the art.The source may supply any signal function, although, for purposes .ofillustration, the sources shown herein will generally be step-functiongenerators. The indicator 1S may be a direct reading meter or some formof recording means,.for example, a time base voltage recorder.

In most servo-systems it will be desirable Ato avoid Ythe use of point 7as a monitor point, because the signal is generally weak at this pointand any loading may affect the amplitude of the error signal and alterthe response of the system.

An alternate arrangement is shown in FIG. 1(b) where the first andsecond monitor points 17 and 18, respectively, are located in thefeedback and -aTT loops where the signal strength is relatively high.

In special cases where 13:-1 the first monitor point may he located atthe output 8 of the servo-system and the second monitor point at point17 as in FIG. 1(c). This arrangement requires access to only the inputand output terminals of the servo-system, but is restricted to =lsystems.

An arrangement requiring access to only the servo-system input andoutput terminals with no restriction on is vshown in FIG. 1(d where thefirst monitor point 8 is located at the servo-system'output and thesecond monitor point is located between the MT and -T portions of path413. This particular monitoring arrangement permits determination of thecorrespondence between a and ,LT and between and T, vas differentiatedfrom the previously described monitoring arrangements, which provide acomparison of the loop gain product a and the product M'r'rln practice,it is highly desirable to have available and make vuse of the extensivedevelopment of D C. analog computers in the construction of themultiplying arrangement 9. In addition, the monitoring means readilyavailable is frequently a varying D C. indicator. Only in specialinstances, however, are the servo-system signals transmitted as varyingD.C. throughout the servo loop.

Thus, in general, when D.C. test signals are employed,

auxiliary transducers are required in coupling between the multiplyingarrangement and the servo-system being tested. One example of this is anautomatic frequency control system.

Application of the present invention to an automatic frequency controlsystem is shown in FIG. 4. The auxiliary transducer in this case is atest oscillator 47 the frequency of which varies with the amplitude ofthe D.C. test signal from arrangement 46. The signal thus obtainedcorresponds to a normal input signal R1, which is a frequency modulatedcarrier wave. The system 41 is vshown with the forward transfer functionsplit into two portions 42 and 43 having transfer ratios nl and u2,respectively. The portion 42 includes a frequency discriminator, and thefirst monitor point 45 is located at the output of this discriminatorwhere the signal variations are essentially varying D.C. The multiplierarrangement 46 and monitor structure are designed in the mannerpreviously described in FIG. 1(a), for example. Since lB=l in thisparticular servo-system, the minus ,BT function of .the multiplierequals unity and no structure is needed to supply this function.

An additional portion Si?, having a transfer function from correspondingto the effect due to the introduction of the oscillator 47, is added tothe test apparatus as a compensatory expedient. The value of pom `isdetermined by a preliminary experimental measurement and is setnumerically equal to the product poul divided by al, thus the value of1T and 2T become numerically equal to a1 and p2, respectively. Theportions 48, 49 and 50 are not duplicates of the portions 42, 43 and 47,but are based on D.C. analog functions. The analogs of portions 42, 43and 47 are easily constructed by one skilled in the analog computer artusing .the preliminary measurements of these components.

Since the signal at monitor .point 45 is the error signal v6Tmultiplied'by the function u1, a similar monitor point in the multiplierarrangement 46 is located `at point 51. By passing ythe signal from thispoint through the portion 50 which compensates for the oscillatorcharacteristic, a signal is obtained which is `compared to the signal`from .monitor point 45 to determine whether .r1.1 and ,t2l conform tospecification.

In cases where the servo-system under test has loop signal transmissionas a varying D.C. voltage at Vsome point in the loop, theA arrangementof FIG. 5 Ymay be used to avoid the auxiliary transducers shown in FIG.4. The test signal is inserted by means of an adder type summing point62 inserted in the forward path of servosystem 60 before the monitorpoint 67. As in FIG. 4 the portions 63, :64 and 65 are represented bytheir DC. analogs in the multiplier arrangement 61. In FIG. 5, the input66 may be any of a wide variety of signal types, e.g., kpulse `timeposition, shaft mechanical position, modulated carrier electricalsignal, etc. The application of the test monitoring method shown in FIG.5 requires varying D.C. signal transmission at only one point 67 in theloop. This varying D C. signal point may be in the feedback path or inthe forward path as shown in FIG. 5. During test periods, the normalservo input 66 is assumed quiescent as discussed previously. Testamplitude in this case must be carefully chosen to avoid servo-systemnondlinearity during the test period.

A basic requirement of servo-systems is that the for.

ward path have a source of power which is controlled by the inputsignal. At the output of the forward path the'signal level is greatlyamplified in the absence of a feedback path. The net gain from input tooutput with the feedback loop closed is essentially unity over theuseful frequency range. Feedback paths seldom include amplificationsources. In analoging the open-loop transfer function, therefore, atleast one portion of the multiplier arrangement in the test apparatusmust have gain, i.e., a transfer function greater than unity. It wouldappear, however, that if the test source were capable of supplying asignal of sufficient amplitude, the multiplier arrangementgainrequirement could be eliminated and the multiplier arrangement couldbe formed from passive elements only. An approach to this idea is shownin FIG. 6.

FIG. 6 shows an alternate embodiment of a'multiplier arrangement 70 andtest signal source 74, as used with any of the systems heretoforedescribed, wherein the multiplier arrangement is composed entirely ofpassive elements. In specific cases, portions 72 and 73 of FIG. 6 mustbe combined and the monitoring method of FIG. l(d) will not bepractical, as in the case of a tachometer generator in the feedbackpath. The gain requirements of each of the elements 71-73 in themultiplier arrangement has been reduced by a factor K, an approximationto the highest gain encountered in any of the elements of themultiplier. This produces an overall reduction of gain which can `heoffset by using a test signal which is greater by a factor of K. Thelargest gain found in servo-systems is ordinarily near zero frequencyand in some cases approaches a theoretically infinite value. Analysis ofthe system under test, however, will yield a practical value to which Kmay be limited. In monitori ing with this arrangement it must be notedthat the signal at monitor point 75 has a different value than thearrangements employing active elements. Thus when arrangements of thetype shown in FIG. 4 are substituted for those in FIG. l(a) the point 76is used in place of point 75 for a second monitor point.

A test signal source which may be employed in the test apparatus isshown in FIG. 7. This source has special utility in testing certaintypes of servo-systems which are to remain in service while under test.In this instance the test signal must be of such a form that it does notinterfere with the signals already present in the system. For example,the chemical process control there are servos which are subjected toltransients similar to step functions when a batch of chemical issuddenly introduced. These transients must be coped with during thenormal operation of the system. If the servo-system test input signal isof exactly lthe same form as the normal input transients, testing may beallowed. Use of the means shown in FIG. 7 permits performance of thetest with open-loop transfer function determination within the systeminput limitation separately prescribed.

In FIG. 7 there is shown a test signal source which will, in combinationwith the previously described multiplier arrangements, reproduce at theservo-system input the waveform of a signal from a preselected source.The test source comprises the preselected source 81 of signals HT havingthe characteristics desired for the servo-system test input signalconnected to the input 82 of a loop 83 containing portions 84 and 85having transfer ratios G and ,8G which are duplicates or analogs of theforward and feedback paths inthe servo-system under test. A signal inthe loop which is equivalen-t to the error signal of the servo-systemunder test is extracted at point 86 and applied to the input of any ofthe multiplier arrangements of the invention. The signal at point 86 isG'T 1-LGG and replaces the signal 0T in the previously disclosedembodiments. After passing through the l--aT/ST multiplier the signalagain becomes @'T at the input of the servo under test provided uGG isexactly equal to, ,aT/8T. Thus the choice of BT generator 81 determinesthe form of the input signal to the servo-system under test.

FIG. 8 shows a servo-system 100, having a transfer function typical of aradar range tracking system, constructed on a standard analog computer.The transfer functions l/.T and T of FIG. 8 were designed to conform tothe asymptotic segment combination, shown in FIG. 9 (a), with a cornerfrequency of 0.5 radians per second. FIG. 9(b) is a generalizedasymptotic representation of the servo-system of FIG. 8. To obtainactual values of wo and wz, the scales shown in FIG. 9(a) must be placedso that the actual corner frequency of the servo-system coincides withwo and the indicated gain at this frequency coincides with gain producedby the system. The analog of these functions was constructed accordingto standard analog computer techniques.

Amplifiers 90, 91 and 92 in FIG. 8 are operational amplifiers in theanalog computer. In this particular embodiment resistors 93, 94, 95 and96 are one megohm resistors; resistor 97 has a value of 0.5 megohm andresistor 98 has a value of 2 megohm. Capacitors 99 and 101 have a Valueof l microfarad. Passive elements 94-99 and 101' also have values equalto their counterparts 94-99 and 101 in the servo-system. The testmonitor summing point 112 comprises two operational amplifiersinterconnected in the manner indicated in FIG. 3. The test signal sourcecomprises a source of D.C. voltage 105 applied across a voltage divider106 through a one megohm resistor 108 to operational amplifier 109.Switch 107 is used to key the D.C. voltage input and provide a stepfunction test signal. A 0.1 megohm resistor 110 is used to provide anegative feedback for amplifier 109 and in combination With inputresistor 108 provides a gain of 0.1.

Amplifiers 91 and 92 form the forward path or u portion of theservo-system. The feedback is a direct connection 114 from output toinput. The test signal from amplifier 109 is applied to the input 104 ofthe multiplier arrangement and one input terminal 111 of the monitorsumming point. The output 116 of the multiplier arrangement is appliedto the input 102 of the servo-system. The error signal from point in theservo-system is connected to the remaining input terminal 117 of themonitor summing point 112. The output of the monitor summing point 113is then fed to a time base recorder 118.

FIG. l0 shows a matrix of time base recordings taken at the output ofthe monitor in FIG. 8. At the center of the matrix is shown the steadyor null output which indicates that the transfer functions of theservo-system have their specification or correct values. The recordingsabove and below the center were made after altering the elements of theservo-system to produce departures from the correct gain, as indicated,and the recordings to the left and right of center were made afteraltering the elements to produce departures from the correct cornerfrequency, as indicated. This is accomplished by placing the scales fromFIG. 9(a) on FIG. 9(b), as previously indicated, and determining the newvalues of the elements in accordance with the same standard analogcomputer technique. Note that the polarities of the test monitor outputsignal are opposite for high and 4low gain, and that the same is truefor departures from the correct corner frequency when the gain iscorrected.

FIG. ll shows the same servo-system with a test signal generator of thetype previously described in FIG. 6 containing only passive elements. Ananalysis of this system indicates that a D.C. gain of 1000 is asufficient approximation of the theoretical infinite gain indicated inFIG. 9a. The values of the various components shown are as follows;resistor 120 is 0.320 megohm, resistor 121 is 0.300 megohm, resistor 122is 0.020 megohm, resistors 123, 124 and 125 are any large value, forexample, one megohm, and capacitors 126 and 127 are l0() microfarads.The relative value of the summing resistor 128 as compared to theisolating resistors 129 and 130 was chosen to give the desired errorvoltage step from the available D.C. voltage appearing across voltagemonitor 133. The power supply of the analog computer provides a sourceof 300 v. D.C. to produce a required error step voltage of 0.1 v. inthis case. Thus the value of K defined in the discussion of FIG. 6 couldhave been as high as 3000 for the specific system of FIG. 8.

Switch 131 is used to open the circuit during the adjustment of thevoltage across meter 133 by means of potentiometer in the unity functionpath. Switch 132 is used to produce the test signal. Note the doubleintegrator using capacitors 126 and 127, which is similar to those inFIG. 8 using amplifiers 91 and 92. The test results obtained with thiscircuit did not differ noticeably from those shown in FIG. 10.

As is evident from the test data in FIG. l0, the present method employssimple time domain test signals as employed in transient testing and yetsupplies information in the frequency domain, the latter being usedextensively in the design of servo-systems. Data concerning theopen-loop transfer function has been obtained in the past by open-loopfrequency response tests which were tedious and time consuming, due tothe extremely low frequencies involved and instrumentation difficulties,as well as inaccurate due to the erratic response (balance drifts, etc.)of some systems in the open-loop condition. The duration of a singletest set forth in the present specification need not exceed 5 to 10system time constants (reciprocal radian bandwidths).

As has been shown, the method and apparatus set forth herein can beadapted to any type system, and requires a minimum amout of structure intheir application. IThe testing apparatus can be built into theservo-systems, if desired, to permit periodic or continuous checking 'inthe field. No unusual access to the system .under testis 4required, andthe system may be tested in some cases even while it is in normalservice. Persons having limited technical skill, supplied withcomparative data, can easily run go-no-go tests on complicated systemsfor quality control. In some cases the components of one servo-systemcan be used to generate a test signal for testing a similar system bycombining these components in the manner described herein.

Other applications and embodiments of the principles and structuredefined herein will be readily devised by those skilled in the art, andthe present invention is therefore to be limited only by the appendedclaims.

What is claimed is:

l. A test apparatus for a servo-system wherein the system has a firstinput summing point and an output, a forward path connected between saidfirst input summing point and said output having an input over outputtransfer function n, a feedback path connecting said output to saidinput summing point having a transfer function and a first monitor pointlocated in one of said paths from which a first monitor signal can besampled, said test apparatus comprising; a test signal source, amultiplier means for multiplying said test signal by l-f/.T-F, where /LTand T are substantially equal to ,u and respectively, said multiplierincluding an input, an output and at least one path therethrough havinga second monitor point from which a second monitor signal can besampled, monitor means for comparing the amplitude of said first monitorsignal with the amplitude of said second monitor signal, a first meansconnecting the input of said multiplier means to said test signalsource, a first signal coupling means interconnecting the output of saidmultiplier means and said first input summing point, whereby a systemsignal is induced in said servo-system at said first monitor point whichcorresponds to a monitor test signal in said multiplier at said secondmonitor point, said monitor means being coupled to said rst monitorpoint, and a coupling path interconnecting said monitor means and saidsecond monitor point, said coupling path having the same transferfunction as said first signal coupling means whereby the open looptransfer function of the servo system may be evaluated without openingthe feedback loop.

2. The apparatus according to claim 1 wherein said test signal Sourcecomprises, a second servo-system having a forward path with a transferfunction LG and a feed- -back path with a transfer -function G, where,MG and G are equal to ILT and T, respectively, the input of saidmultiplier means being connected to the forward path of said secondservo-system.

3. The apparatus according to claim 1 wherein said second monitor pointis located at the input of said multiplier means.

4. The apparatus according to claim 1 wherein the multiplier -meanscomprises a first and a second path, each having an input and an output7`said first path containing means for multiplying any signal therein bya factor ,it-DST, said test signal source being connected to the inputof each path, and a second summing -point for combining the signals insaid first and second paths connected to the output of each of saidpaths.

5. The apparatus according -to claim 4 wherein the second monitor pointis located at the -output end of said first path.

6. The apparatus according to claim 4 wherein said first path comprisestwo serially connected portions, a first of said portions having atransfer ratio of [LT and a second of said portions having a transferratio of 13T.

7. The apparatus according `to claim 6 wherein said sec- '11@ ondmonitor -point is located -between said first and second portions.

8. `Combined servo-system and test apparatus comprising; a vservo-systemincluding a rst input summing point Jto which a source of input testsignal is connected and an out-put, a forward `path connecting saidinput summing point to said output characterized by a transfer functionn, a feedback path connecting said output to said input summing pointcharacterized by a transfer function ,8, a first monitor point locatedin one of said forward and feedback paths, multiplier means connectedbetween said source and said input summing point for multiplying inputsignals app-lied thereto by 1-;1.T[3T, where ,LT and T are substantiallyequal to y. and whereby a system signal is induced at said first monitorpoint which corresponds to a monitor test signal in said multipliermeans at a second monitor point, and monitor means connected to saidfirst monitor point for comparing the system signal induced at saidfirst monitor point by said test signal with said monitor test signalsimilarly induced in said multiplier means, whereby the open looptransfer function of the servo-system may be evaluated without openingthe feedback loop.

9. A test apparatus according to claim 8 wherein said first monitorpoint is located in said forward path adjacent to said first summingpoint.

10. A test apparatus according to claim 8 wherein said rst monitor pointis located in said feedback path adjacent said first summing point.

1l. A test apparatus according to claim 8 wherein said first monitorpoint is located at the output of said servosystem.

l2. A test apparatus `for a servo-system, wherein said servo-system hasa source of modulated carrier waves connected at the input thereof and adetector therein for deriving a varying D.C. signal from said modulatedcarrier, comprising; a test signal source, D.C. analog means connectedto said source for multiplying the output of said source by l-,uT-f,where /iTT is substantially equal to the open loop transfer function ofthe servo-system, an oscillator having the same frequency as saidcarrier, said oscillator having a D.C. input for modulating saidoscillator, means connecting the output of said D.C. analog means tosaid D.C. input, means connecting the output of said oscillator to theinput of said servo-system, whereby a system signal is induced in saidservo-system at the detector output which corresponds to a monitor testsignal induced in said D.C. analog means and monitor means connected tothe output of said dete-ctor for comparing the amplitude of a signalfrom said detector with the amplitude of a signal in said D.C. analogmeans, whereby the open loop transfer function of the servo-system maybe evaluated without opening the feedback loop.

13. A test apparatus for a servo-system, wherein the servo-systemincludes at least one component therein which has an output providing avarying D.C. signal; comprising a vsource of test signal, D.C. analogmeans for multiplying said test signal by l-pTT connected to saidsource, where nTT is substantially equal to the open loop transferfunction of the servo-system, a summing point having at least two inputsand an output interposed in the servo-system with a first of said inputsconnected to the output of said component, the output of said D.C.analog means being connected to a second input of said summing point,whereby a system signal is induced in said servo-system corresponding toa monitor test signal induced in said D.C. analog means, and monitormeans connected to the output of said summing point `for comparing theamplitude of signals from said summing point with the amplitude of asignal in said D.C. analog means, whereby the open loop transferfunction of the servosystern may be evaluated without opening thefeedback loop.

(References on following page) References Cited in the le of this patentUNITED STATES PATENTS OTHER REFERENCES Liu et al.: Extending TransducerTransient Response by Electronic Compensation for High-Speed PhysicalMeasurements, Review of Scientific Instruments, voi. 29, No. 1, January,1958; pages 14-22.

